1. Field of the Invention
The present invention relates generally to a pole change induction motor and control apparatus and method for controlling the same which is applicable to, for example, a driving source of an electric vehicle with no power transmission.
2. Description of the Background Art
Frequency (rotation speed) controls with inverters as power supplies for induction motors have been adopted as variable speed controls of induction motors. A torque characteristic of a torque with respect to a frequency (revolution or rotation speed) of the motor indicates a constant torque characteristic (dotted line with V in FIG. 1) so that the supplied voltage to the motor is, at this constant torque range, increased in proportion to the increase in the rotation speed. (The constant torque characteristic is such that even if the frequency (revolution speed) of the motor is changed, the torque of the motor does not change.)
FIG. 1 shows a revolution speed (frequency) versus torque characteristic graph in a frequency control of the induction motor.
As denoted by a solid line of FIG. 1, in a speed range in which a constant output is derived from the motor (hereinafter referred to as a constant output driving range) and which is in excess of the constant torque characteristic, the torque T is reduced along a curved line of 1/f (f: frequency of the motor) which is generally inversely proportional to the increase in the frequency due to a limitation of the applied voltage V from the inverter to the induction motor. A broken line shown in FIG. 1 denotes a maximum torque characteristic T' that the induction motor naturally has. At frequencies higher than a frequency denoted by af in FIGS. 1 and 2 (f in this case denotes a base speed) over which a point of intersection between the maximum torque characteristic curve T' (T' is proportional to 1/f.sup.2) and the torque characteristic T (T is proportional to 1/f) is present, the output torque of the motor is limited by the maximum torque characteristic T'. In FIGS. 1 and 2, a denotes an arbitrary number (integer) generally greater than 2. However, even if a is 1 or more, the above-described limitation of the output torque is established.
When the induction motor is driven in the constant output (characteristic) range shown in FIG. 1, the output torque is reduced as the frequency (rotation speed) becomes higher. In addition, the output torque is limited according to the maximum torque characteristic of the induction motor so that an insufficient torque is resulted according to a load applied to the induction motor.
The following driving methods have conventionally been considered in order to eliminate the torque insufficiency problem described above.
(1) Production of a large capacity and large dimension induction motor. If such an induction motor as having a maximum torque characteristic T" shown in FIG. 2 is used, the torque from the torque characteristic T shown in FIG. 2 can be derived until the rotation speed is increased and reaches to a predetermined speed (frequency) 2af according to the alternative maximum torque characteristic T" in place of the maximum torque characteristic T' as shown in FIG. 2.
(2) Production of a large capacity and large dimension inverter. If an output power (capacity) of the inverter connected to the induction motor is increased, the output voltage of the large dimension inverter is changed from V1 to V2 shown in FIG. 3 so as to derive the maximum torque characteristic T1" from that T1' of the induction motor, thus the output torque until the frequency (RPM) 2af being assured.
(3) Mechanically or manually switching of connections between windings in the induction motor: Such a driving of the induction motor as described below is carried out. That is to say, a pole change (change or switching, namely, the number of poles are changed) is carried out by mechanically or manually switching the connections between winding switching terminals of the induction motor and inverters when the driving revolution speed is in excess of a certain range.
However, in the above-described item (1), to obtain the double frequency (2af/af=2) as the value of the increased torque, it is necessary to double the dimension of the inverter so that the large sized whole system including the double sized induction motor is resulted.
In the above-described item (2), if the torque increase is resulted, an output current of the inverter is increased due to the change in a voltage characteristic applied to the induction motor and, thus, the whole system including the large-sized inverter itself is increased.
In the above-described item (3), it is necessary to provide a switch to change the number of poles. In addition, a switching operation interval of the switch corresponds to an idling time duration (no load driving) of the motor driving so that a torque shock (variation) due to an abrupt change of the output torque occurs during this switching operation. Furthermore, due to a frequent use of the switch causes a shortened life of the switch or failure in the switch.